The present invention is directed to a circuit for operating a high intensity discharge (HID) lamp. More particularly, the circuit is for automotive lamp applications.
HID lamps sense the current in the lamp to regulate lamp power. A known technique for sensing current is to connect a resistor in series between ground and the lamp (see, for example, U.S. Pat. Nos. 4,170,747 and 6,389,694). In such lamps, during lamp run-up the current in the lamp is limited to, for example, 2.5 amps for lamps containing mercury and 4 amps for mercury-free lamps. If a 0.68 ohm resistor is used to sense lamp current, there is a 1.7 volt drop across the resistor. Since this voltage drop is felt in the field effect transistors (FETs) of the H-bridge lamp driver connected to ground, a level shift circuit must be added to drive these FETs.